At the present time there is a large interest in transmitting cable television signals along an optical fiber lightwave transmission system. The format most commonly used to transmit cable television (CATV) signals along an optical fiber is amplitude modulated vestigial sideband (AM-VSB) where the cable television electrical signal is intensity-modulated onto an optical signal. In practice, severe limitations are normally placed on laser noise and linearity requirements.
External modulation of continuous wave (CW) lasers provides a source which is free of second harmonic distortion, one of the major causes of nonlinearity. CW lasers also have a compact optical spectrum which effectively ameliorates distortion due to fiber chromatic dispersion. Unfortunately, they can enhance the effect of optical nonlinearities, particularly stimulated Brillouin scattering (SBS).
The latter converts the transmitted signal in an optical fiber to a backward scattered signal. Thus, SBS sets a limit on the total amount of optical power that can be injected into an optical fiber.